Project Summary (Overall) The purpose of this renewal application is to continue the successful activities of our center, which uses quantitative genetic techniques to study the genetic basis of drug abuse-related behaviors in outbred rats. When our center was initially funded in June 2014, our goal was to develop outbred N/NIH heterogeneous stock (HS) rats as a platform for genetic studies of behaviors that were difficult or impossible to study in mice. The first four years of funding have allowed us to establish a vibrant community of investigators using HS rats to study drug abuse and other traits, which we refer to as an ecosystem. This ecosystem includes both the investigators who are directly involved in this renewal application and many others who have obtained separate funding, some from NIDA, and some from other sources. The growth of this ecosystem reflects one of the ways that our center has served as national resource. We are proposing three projects that involved phenotyping HS rats for a variety of traits, including intravenous cocaine and nicotine self-administration, response to novelty, social behavior, reaction time, and delay discounting. Two of those projects are continuations from the prior funding period and are designed to increase our sample size from 1,600 to 3,200 rats per phenotype. We present data showing that such an increase produces an exponential increase in the number of significant findings. This approach parallels human genetics studies of SUD, which have also benefited tremendously from larger sample sizes. We will use these data to conduct genome-wide association studies (GWAS) and a suite of related techniques. In addition, we will measure gene expression in behaviorally nave rats using RNASeq and use those data to identify expression quantitative trait loci (eQTLs). We will then integrate GWAS and eQTL data in an effort to identify specific genes that influence the behavioral phenotypes. Many of the behavioral domains being studied are known to be sexually dimorphic; our study will use both male and female rats, which will allow us to identify sex differences and sex by genotype interactions. We will also study genetic correlations, perform phenome-wide association studies (PheWAS), transcriptome wide association studies (TWAS) and explore a novel strategy called polygenic transcriptomic risk scores (PTRS), that is intended to allow translation of polygenic signals across species. Project 4 will use a network-based approach to extend our GWAS to account for known biological networks. This proposed renewal also includes a pilot project core to support new directions and take advantage of unforeseen opportunities. Finally we propose an administrative core that supports many activities of the center, including educational, career development and public outreach. The results of these studies will enhance our understanding of the role of genes in a range of psychologically complex behaviors and will provide novel biological insights that may support future efforts at preventing or treating drug abuse.